"He reached for the brass doorknob but suddenly withdrew his hand as though he'd been burned. 'What's the matter?' said Harry. 'Can't go in there,' said Ron gruffly. 'That's a girl's toilet.' 'Oh, Ron, there won't be anyone in there,' said Hermione. 'That's Moaning Myrtle's place. Come on, let's have a look.'"

This girls' toilets is situated on the first floor of Hogwarts Castle, just above the Great Hall.[1] It has been out of order ever since a student named Myrtle was killed there. Myrtle has haunted the bathroom ever since the Ministry of Magic stopped her from haunting Olive Hornby who had been teasing Myrtle just before her death, leading it to be a place most students do not want to enter. This bathroom holds the entrance to the Chamber of Secrets. To get in the Chamber, a person needs to say "open" in Parseltongue. This will cause the tap with a snake on the side (a tap which incidentally, never worked) to move into the floor, revealing a tunnel which leads downwards into the Chamber.

Myrtle's ghost

After her death, Myrtle's ghost (or Moaning Myrtle, as the students usually call her) haunted Olive Hornby, the fellow student who caused her to be in the bathroom that evening. When the Ministry of Magic refrained her from doing so, Moaning Myrtle returned to the bathroom, haunting indefinitely.

In the video game adaptation of Harry Potter and the Half-Blood Prince this lavatory is replaced by a male bathroom, with different sinks and urinals instead of stalls. In the movie adaptation, however it can be seen that the corridor outside the male bathroom is different from the corridor outside Myrtle's bathroom.